Conformally coated wire array photoelectrodes for photoelectrochemical fuel generation

ABSTRACT

Embodiments of the present invention are directed to photoelectrodes having a wire array core and a conformal coating on the core. The wire array core and the conformal coating can be independently selected from inorganic semiconductor materials. The photoelectrodes can be used as either or both the anode and cathode in a device for fuel generation. Such a device, for example, could include a photoanode and a photocathode separated from each other by an electrically and ionically permeable, and proton-conductive membrane.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to and the benefit of U.S. Provisional Application Ser. No. 61/330,216, filed on Apr. 30, 2010, entitled CONFORMALLY COATED WIRE ARRAY PHOTOELECTRODES FOR PHOTOELECTROCHEMICAL FUEL GENERATION, the entire contents of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The Federal government has certain rights in this invention pursuant to Grant No. W911NF-09-2-0011 awarded by the Defense Advanced Research Projects Agency (DARPA).

TECHNICAL FIELD

The invention is directed to wire array photoelectrodes for photoelectrochemical fuel generation, to methods of generating solar fuel, and to devices for generating solar fuel, e.g., syngas.

BACKGROUND

To provide energy storage and liquid fuel for transportation and other applications, it is desirable to have a method to convert solar energy, often along with water and/or CO₂, into energy-containing products such as methanol or synthesis gas (“syngas”), i.e., a mixture of CO and H₂. Syngas can be easily converted into various liquid fuels via the Fischer-Tropsch process. These processes can be used to recycle carbon dioxide and/or water back into useful fuel, with the only energy input being solar radiation.

The production of synthesis gas from sunlight requires four conceptually distinct processes: 1) The sunlight must be captured and converted into separated electrical charges, producing a photocurrent with sufficient photovoltage to drive the endoergonic fuel formation involved with production of CO and H₂ from H₂O and CO₂; 2) H₂O must be reduced to produce H₂; 3) CO₂ must be reduced to produce CO; and 4) water must be oxidized to form O₂. The fourth process, oxidation of water to form O₂, is required in order to have the only steady-state inputs into the system be sunlight, CO₂ and H₂O, per the following balanced chemical equations:

$\begin{matrix} {\quad\begin{matrix} {{h\; \upsilon} = {e^{-} + h^{+}}} & {\Delta \; E} \\ {{{2H^{+}} + {2e^{-}}} = H_{2}} & {E^{0} = {0.00\mspace{14mu} V}} \\ {{{CO}_{2} + {2H^{+}} + {2e^{-}}} = {{CO} + {H_{2}O}}} & {E^{0} = {{- 0.10}\mspace{14mu} V}} \\ {{2\; H_{2}O} = {O_{2} + {4H^{+}} + {4e^{-}}}} & {E^{0} = {1.23\mspace{14mu} V}} \\ {{{CO}_{2} + {H_{2}O} + {h\; \upsilon}} = {{CO} + H_{2} + O_{2}}} & {E^{0} = {1.33\mspace{14mu} V}} \end{matrix}} & \begin{matrix} \begin{matrix} \begin{matrix} (1) \\ (2) \end{matrix} \\ (3) \end{matrix} \\ (4) \end{matrix} \end{matrix}$

The key processes are represented by Equations (1)-(3), and these processes determine the fuel that will be produced. The fourth process represented by Equation (4) sets a lower bound on the thermodynamically required photovoltage needed to drive the chemical reactions of concern, and is a constraint that must be satisfied in order to insure that the only sustainable chemical inputs into the system are CO₂ and H₂O.

A typical approach to the conversion of solar energy has been the use of planar, single crystal wafers of a semiconductor, such as gallium phoshide (GaP), often in conjunction with a suitable catalyst, to absorb incident sunlight, generate an electron-hole pair, and donate these carriers into solution, where they drive a chemical reaction to reduce CO₂ either in solution or in vapor. Within this planar, single crystal wafer methodology, research has generally focused on the use of nanoparticle or powder samples to enhance the surface area at which the reaction can occur and/or decrease the cost of the material.

However, in these traditional planar junctions, the directions of light absorption and of charge-carrier collection are mutually parallel, as shown in FIG. 1 a. Charge-carriers generated deep within the bulk of the absorbing semiconductor material must therefore necessarily traverse the full thickness of the absorber to be collected. Accordingly, the requisite, long, minority-carrier diffusion length, L, explicitly demands high material purity to minimize bulk carrier recombination, and thus implicitly requires significant materials preparation and purification costs:

SUMMARY

Embodiments of the present invention are directed to substrates comprising a core including a wire array comprising a first inorganic semiconductor material, and a conformal coating on the wire array core, the conformal coating comprising a second inorganic semiconductor material. Each of the first and second inorganic semiconductor materials can be independently selected from Group IV elemental semiconductors, Group IV compound semiconductors, III-V semiconductors, II-VI semiconductors, and oxide semiconductors. For example, the first inorganic semiconductor material may comprise Si, Ge, a SiGe alloy, GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy or WO₃. Also, in some embodiments, the second inorganic semiconductor material may be different from the first inorganic semiconductor material and may comprise GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃. In some exemplary embodiments, the first inorganic semiconductor material is Si, Ge or a SiGe alloy, and the second semiconductor material is GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃. For example, in some embodiments, the first inorganic semiconductor material is Si, and the second semiconductor material is GaP. The wires of the array may have a length of greater than about 100 nm.

According to other embodiments of the present invention, a device comprises a first electrode, a second electrode, and a membrane separating the first electrode and the second electrode. The first electrode comprises the above-described photoelectrode. In particular, the first electrode comprises a wire array core comprising a first inorganic semiconductor material, and a conformal coating on the wire array core, the conformal coating comprising a second inorganic semiconductor material. Each of the first and second inorganic semiconductor materials can be independently selected from Group IV elemental semiconductors, Group IV compound semiconductors, III-V semiconductors, II-VI semiconductors, and oxide semiconductors. For example, in some embodiments, the first inorganic semiconductor material comprises Si, Ge, a SiGe alloy, GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃. In other exemplary embodiments, the second inorganic semiconductor material is different from the first inorganic semiconductor material and comprises GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃. In still other exemplary embodiments, the first inorganic semiconductor material is Si, Ge or a SiGe alloy, and the second semiconductor material is GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃. According to some exemplary embodiments, the first inorganic semiconductor material is Si, and the second semiconductor material is GaP. In some alternate embodiments, the wires of the array have a length of greater than about 100 nm.

In some exemplary embodiments, the second electrode comprises a second wire array core and a second conformal coating on the second wire array core. The second wire array core comprises a third inorganic semiconductor material, and the second conformal coating comprising a fourth inorganic semiconductor material. In some exemplary embodiments, the first electrode comprises a photoanode, the first inorganic semiconductor material of the first electrode comprises Si, and the second inorganic semiconductor material of the first electrode comprises n-type GaP. Also, in this embodiment, the second electrode is a photocathode, the third inorganic semiconductor material of the second electrode is Si, and the fourth inorganic semiconductor material of the second electrode comprises p-type GaP.

According to some embodiments, the first electrode is a photocathode, and the second electrode is a photoanode and comprises porous WO₃. The porous WO₃ may comprise an array of porous WO₃ wires. Alternatively, in some embodiments, the first electrode is a photoanode, and the second electrode is a photocathode and comprises an array of Si wires.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, in which:

FIG. 1 a is a schematic depicting the light absorption pathlength of a depth (1/α) and minority charge-carrier collection length, L, in semiconductor photovoltaic/photoelectrochemical junctions with planar geometries, according to the prior art;

FIG. 1 b is a schematic depicting the light absorption pathlength of a depth (1/α) and minority charge-carrier collection length, L, in semiconductor photovoltaic/photoelectrochemical junctions with rod geometries, according to embodiments of the present invention;

FIG. 2 a is a scanning electron microscope (SEM) image of a silicon rod array taken from side-on (Scale bar: 15 μm);

FIG. 2 b is a SEM image of a silicon rod array taken from top-down (b) view (Scale bar: 86 μm);

FIG. 3 a is an optical photograph of a freestanding Si rod array embedded in PDMS;

FIG. 3 b is an SEM image of the top of a Si/PDMS composite film;

FIG. 3 c is an edge-on view of a Si/PDMS composite film;

FIG. 4 a is an SEM image of a CdSe_(x)Te_(1-x) rod array;

FIG. 4 b is a graph of the spectral response of a CdSe_(x)Te_(1-x) rod electrode versus a planar electrode immersed in aqueous solution containing 1 M NaOH, 1 M Na₂S, and 1 M S;

FIG. 5 is a schematic comparing the flux of photogenerated charge carriers across a planar junction (left) and a rod junction 9 (right) under illumination;

FIG. 6 is a diagram of the band energetics in a dual photocatalyst system;

FIG. 7 is a schematic of a solar powered water-splitting device, according to an embodiment of the present invention, incorporating two separate semiconductor rod-array photoelectrodes that sandwich an electronically and ionically permeable membrane;

FIG. 8 is a graph comparing the band diagrams of photoelectrode materials depicted with respect to the reduction and oxidation potentials of H₂O at pH=0;

FIG. 9 is a schematic of a photocathode according to an embodiment of the present invention including p-Si rods decorated with particulate metal electrocatalysts;

FIG. 10 is a schematic of a photoanode according to an embodiment of the present invention including n-WO₃ rods decorated with particulate metal oxide electrocatalysts;

FIG. 11 is a schematic depicting two methods for the electrodeposition of WO₃-alloy rod arrays according to embodiments of the present invention;

FIG. 12 a is a diagram of a simulated two-dimensional GaP/Si unit cell according to embodiments of the present invention;

FIG. 12 b is a graph showing the simulated absorption of the simulated two-dimensional GaP/Si “grating” shown in FIG. 12 a;

FIG. 13 a is a diagram of a simulated three-dimensional GaP/Si unit cell according to embodiments of the present invention;

FIG. 13 b is a graph showing the simulated absorption of the simulated three-dimensional GaP/Si wire array shown in FIG. 13 a;

FIG. 14 is a graph depicting normal incident absorption of a GaAs/Si wire array according to embodiments of the present invention;

FIG. 15 a is a contour plot assuming Beer-Lambert absorption of the efficiency of a simulated GaP/Si wire array according to embodiments of the present invention;

FIG. 15 b is a contour plot assuming Beer-Lambert absorption of the open circuit voltage of a simulated GaP/Si wire array according to embodiments of the present invention;

FIG. 15 c is a contour plot assuming Beer-Lambert absorption of the short circuit current of a simulated GaP/Si wire array according to embodiments of the present invention;

FIGS. 16 a and 16 b are SEM images of a GaP-coated Si wire array according to embodiments of the present invention, taken at different magnifications;

FIG. 16 c is an SEM image of a cross-section of a rod of a GaP-coated wire array cleaved in the horizontal direction according to embodiments of the present invention showing the Si core and GaP coating;

FIG. 16 d is an SEM image of a cross-section of a rod of GaP-coated wire array cleaved in the longitudinal direction according to embodiments of the present invention;

FIG. 16 e is an SEM image of a GaP-coated Si wire array having branched wires according to embodiments of the present invention;

FIG. 17 a depicts the X-ray diffraction measurements (ω-2θ scans) of wire array GaP/Si, planar GaP/Si, and Si samples;

FIG. 17 b depicts the X-ray diffraction measurements (rocking curves) of GaP/Si, and Si samples;

FIG. 17 c is a reciprocal space map centered around the <111> peak of a GaP/Si wire array according to an embodiment of the present invention;

FIGS. 18 a through 18 c are TEM images of cross sections of GaP/Si wires according to embodiments of the present invention;

FIG. 18 d is a selected area diffraction pattern of a Si core taken along the <111> zone axis;

FIG. 18 e is a selected area diffraction pattern of a GaP shell taken along the <111> zone axis;

FIG. 19 is a graph comparing the photoluminescence spectra from a planar n-type GaP on Si sample, an n-type conformally GaP coated Si wire sample, and a commercial Zn-doped GaP wafer;

FIG. 20 a depicts the optical absorption of peeled-off Si wire arrays (Si wires only);

FIG. 20 b depicts the optical absorption of peeled-off GaP-coated Si wire arrays; and

FIGS. 21 a through 21 d depict steps in a method of fabricating a conformally coated wire array structure (along with SEM images of the substrate after each step) according to embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention are directed to photoelectrodes for use in photoelectrochemical fuel generation. For example, the inventive electrode structures may be used and/or adapted or modified to work in photovoltaic or other solar cell applications. In some embodiments, the photoelectrodes include a vertically oriented wire array core made of an inorganic semiconductor material, and a conformal coating on the wire array. As used herein, the term “conformal coating” refers to the geometry of the coating with respect to the wire array core, i.e., a “conformal coating” is one that conforms to the geometry of the array. In particular, as shown in FIGS. 12 a, 13 a, 16 c and 16 d, the “conformal coating” tracks the rod geometry of the wire array such that the conformally coated wire array photoelectrode maintains the rod geometry of the core (or uncoated) wire array. In some embodiments of the present invention, the material of the core wire array may be an inorganic semiconductor material that is capable of being more easily grown (e.g., by vapor-liquid-solid growth or other method) in the wire array geometry, while the material of the conformal coating ma be a material that is desirable to photoelectrochemistry but that is not as easily grown in the wire array geometry by itself.

As discussed above, the material of the wire array includes an inorganic semiconductor material. Indeed, any inorganic semiconductor material that is capable of being grown in a wire array geometry may be employed. Some nonlimiting examples of suitable inorganic semiconductor materials include Group IV elemental materials, Group IV compound materials (i.e., IV-IV materials), III-V materials, II-VI materials, I-VII materials, IV-VI materials, V-VI materials, II-V materials, oxide materials, layered materials, magnetic materials, and charge-transfer complexes. As used herein, the notation “III-V” and the like denotes the components of the semiconductor material, such that a III-V material is a material that includes at least one Group III element and at least one Group V element. In some embodiments, for example, the inorganic semiconductor material of the wire array my be a Group IV elemental semiconductor, a Group IV compound semiconductor, a III-V semiconductor, or an oxide semiconductor. Some nonlimiting specific examples of suitable inorganic semiconductors for the material of the core (or uncoated) wire array include Si, Ge, SiGe alloys, GaP, GaAs_(x)P_(1-x) alloys, GaAs_(x)N_(y)P_(1-x-y) alloys and WO₃. In some embodiments, for example, the material of the core wire array is Si.

The vertically oriented wire array cores can be fabricated by any suitable method. In some embodiments, for example, the wire array cores can be fabricated by vapor-liquid-solid growth. This method of fabricating wire array cores is described in Spurgeon, et al., “Repeated epitaxial growth and transfer of arrays of patterned, vertically aligned, crystalline Si wires from a single Si(111) substrate,” Applied Physics Letters, 93, 032112 (2008), and U.S. Patent Application Publication Nos. 2009/0020150 to Atwater, et al., entitled STRUCTURES OF ORDERED ARRAYS OF SEMICONDUCTORS and published Jan. 22, 2009, and 2009/0020853 to Kayes, et al., entitled STRUCTURES OF AND METHODS FOR FORMING VERTICALLY ALIGNED SI WIRE ARRAYS and published Jan. 22, 2009, the entire contents of all of which are incorporated herein by reference. In some embodiments, for example, the vertically oriented wire array core is grown by first applying a buffer oxide (e.g., silicon oxide) on a planar substrate wafer, patterning the buffer oxide layer on the planar substrate wafer (e.g., a Si wafer) with, for example, a photoresist, removing the portion of the buffer oxide in the holes generated by the photoresist pattern, applying a catalyst by, for example evaporation of a metal catalyst (e.g., Au or Cu) to the resulting patterned substrate wafer (See FIG. 21 a), and removing the photoresist (by, e.g., lift-off). As shown in FIG. 21 a, the resulting substrate wafer includes a patterned buffer layer having holes in which the catalyst particles (shown as light gray bumps in the SEM image) are situated. This resulting substrate wafer is then annealed in a H₂ environment, and then a growth material (for example, chlorosilane, i.e., SiCl₄) is introduced to grow the wires on the wafer underneath the metal catalyst particles (see FIG. 21 b). In some embodiments, for example, the annealing temperature can be about 1000 C., and be carried out for about 20 minutes under 1 atm of H₂ at a flow rate of about 1000 standard cm³ min⁻¹ (SCCM)). Also, in some embodiments for the growth of Si wires, growth of the wires can include about a 30 minute process with addition of the chlorosilane at a flow rate of about 20 SCCM, while maintaining the same pressure, temperature and H₂ flow rate as the annealing process. After growing the wires under the metal catalyst particles, the catalyst and buffer layer are removed to produce a wire array core that is ready for epitaxial growth of the conformal coating (see FIG. 21 c). The metal catalyst particles and buffer layer can be removed by any suitable technique or method, for example, by selective chemical etching. Those of ordinary skill in the art would be capable of selecting a proper chemical for selectively etching the catalyst particles and/or buffer layer.

As noted above, however, the vertically oriented wire array cores can be fabricated by any suitable method. For example, as an alternative to vapor-liquid-solid growth, the wire array cores could be etched from a solid/planar material.

The wires of the wire array cores can have any suitable dimension, including any suitable length and any suitable diameter. The diameter of the wires in the wire array cores can be controlled and modified by altering the size of the holes in the photoresist into which the catalyst particles are situated, and altering the volume of the catalyst deposited in the holes. For example, the optimal catalyst volume can be calculated for a given wire radius from the surface tension of the catalyst meniscus on the growing wire. The size of the holes into which the catalyst particles are placed dictates the diameter of the wires grown during the vapor-liquid-solid growth process.

Also, while any suitable wire length can be obtained by suitable manipulation of the vapor-liquid-solid growth parameters, in some embodiments, the wires have lengths of greater than about 100 nm. In particular, the wire array cores may include microwire arrays, where the term “microwire” as used herein denotes a wire having a length of greater than about 100 nm.

Also, large arrays of wires can be grown according to embodiments of the present invention. In particular, in some embodiments of the present invention, the wire array cores can have large areas, for example of about 1 m². Additionally, the wafers made from conformally coated wire arrays according to embodiments of the present invention can be rather large, for example about 6 inches.

The material of the conformal coating may be any semiconductor material, and may be deposited on the wire array cores by any suitable technique or method. For example, in some embodiments, the material of the conformal coating may be electrodeposited on the wire array, which would result in a polycrystalline material having no preferred orientation. Alternatively, in other embodiments, the conformal coating is epitaxially grown on the inorganic semiconductor material of the wire array. As used herein, the term “epitaxially grown” refers to the procedure for growing the conformal coating on the core wire array, and indicates a growth process by which a film is grown on a seed crystal from gaseous or liquid precursors. As is known to those of ordinary skill in the art, epitaxially grown films take on the lattice parameters of the substrate on which they are grown. Any semiconductor material that is capable of being grown on the core wire array substrate can be used as the material of the conformal coating. Some nonlimiting examples of suitable semiconductor materials for the conformal coating include Group IV elemental materials, Group IV compound materials (i.e., IV-IV materials), III-V materials, II-VI materials, I-VII materials, IV-VI materials, V-VI materials, II-V materials, oxide materials, layered materials, magnetic materials, and charge-transfer complexes. In some embodiments, for example, the inorganic semiconductor material of the wire array may be a Group IV elemental semiconductor, a Group IV compound semiconductor, a III-V semiconductor, or an oxide semiconductor. Some nonlimiting specific examples of suitable semiconductors for the material of the conformal coating include amorphous Si, amorphous Ge, amorphous SiGe alloys, GaP alloys, GaAs_(x)P_(1-x) alloys, GaAs_(x)N_(y)P_(1-x-y) alloys, and WO₃. In some embodiments, for example, the material of the conformal coating is p-type or n-type GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃. Accordingly, in some exemplary embodiments, the photoelectrode includes a core wire array structure comprising Si, Ge or a SiGe alloy, and the a conformal coating comprising p-type GaP, n-type GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃.

After growth of the wire array cores, the conformal coating is epitaxially grown on the wire array cores (see FIG. 21 d). For example, in some embodiments of the present invention, the conformal coating is grown on the wire array cores using metalorganic chemical vapor deposition (i.e., metalorganic vapor phase epitaxy). In this process, metalorganic source gases are used to epitaxially grow the desired conformal coating on the wire array core. This process enables the growth of conformal layers of the coating, i.e., the growth of layers that conform to the geometry of the wires in the wire array cores. The metalorganic source gases are selected based on the desired composition of the conformal coating. For example, if a GaP coating is desired, a Ga source gas (e.g., trimethylgallium) and a P source gas (e.g., phosphine or tert-butyl phosphine) could be used as the source gases for the epitaxial growth process. Also, to make a p-type or n-type conformal coating, the ratios of the source gases can be modified and altered depending on whether a p-type or n-type coating is desired. The selection of suitable source gases for the growth of epitaxial layers, and the modification of the ratios of the source gases to yield and n-type or p-type film is within the skill of those of ordinary skill in this field. For example, in some embodiments, an indium arsenide coating could be grown using trimethyl indium and arsine.

Also, the conformal coating can be applied to the wire array cores by any suitable technique, and is not limited to the epitaxial growth discussed above. For example, the conformal coating may be applied using electrodeposition, sputtering, or sol-gel techniques, or the like.

As discussed above, the conformally coated wire arrays have vertically oriented, wire array geometries. As used herein, the term “vertically oriented” means that the wires in the array are generally oriented in a vertical direction. However, the term “vertically oriented” does not exclude wire array geometries in which the wires in the array have branches. For instance, the vertically oriented, conformally coated wire arrays, according to some embodiments of the present invention, may include branched wires stemming (or branching) from a main, vertical wire, as shown in FIG. 16 e.

These branched wire arrays can be made by substantially the same methods described above with respect to embodiments that are not branched. However, to make the branches in the wires, the flow of one source of the conformal coating material (e.g., the Ga source) is turned on for a short period of time, while the flow of the other source of the conformal coating material (e.g., the P source) is off. This short exposure to the one source material before then turning on the flow of the second source enables self-catalyzed growth of wires of the conformal coating material. In some embodiments, for example, self-catalyzed growth of GaP wires on Si wire array cores is accomplished by turning on the Ga source (e.g., trimethylgallium (TMGa))) for a brief period of time prior to turning on the P source, so that there is a brief period in which the core is exposed only to the Ga source. This method results in the branched, conformally coated wire array structures shown in FIG. 16 e. The branches could also be grown by using an extrinsic catalyst (e.g., gold) dispersed on the core wires prior to shell growth. The branches give higher surface area for catalysis, may have better material quality, and allow for smaller diameter wires in materials with shorter minority carrier diffusion lengths to be used in conjunction with a longer diffusion length core material.

After growth of the conformal coating on the wire array core, the conformally coated wire array can be removed from the substrate, and the substrate can be reused to grow additional wire array cores and conformal coatings. For example, in some embodiments, the conformally coated wire arrays grown on the substrate can be coated in a polymer or similar material (e.g., polydimethylsiloxane (PDMS)), followed by curing of the polymer. The polymer film and the embedded wires of the array can then be removed by scraping the wafer surface with a razor blade or the like. This transfer procedure preserves the pattern fidelity and vertical alignment of the wires within the polymer matrix. After removal of the wire embedded polymer film, the residual wire stubs on the substrate are selectively etched off the substrate, substantially returning the substrate to its original growth state, having the original buffer hole pattern with the substrate exposed at the bottom of the holes. The substrate can then be reused to grow subsequent wire arrays. The polymer embedded wires can then be transferred to another substrate, and the polymer removed.

The vertically oriented, conformally oriented wire array structures according to embodiments of the present invention provide several advantages. For example, the vertical wire geometry provides much larger surface area than planar films, by as much as a factor of 100 or more, depending on the wire length. Higher surface area means that chemical reactions can occur at more sites simultaneously, increasing the reaction rate. Also, in photoelectrochemical applications, vertically oriented wires absorb light along the length of the wire, but carrier extraction occurs in the shorter radial direction. This means that lower quality, and therefore cheaper, materials can be used because the absorption length can be much longer than the minority carrier diffusion length. In addition, in contrast to planar wafers, wire arrays can be embedded in a polymer (or the like) matrix and removed from the substrate, as discussed above, allowing the substrate to be reused and thereby reducing cost.

According to embodiments of the present invention, the conformally coated wire array structures could be used to drive a number of reactions. For instance, in some embodiments, the conformally coated wire arrays could be used to convert CO₂ to CO, H₂ and O₂. Also, in some embodiments, the conformally coated wire arrays could be used to split water into H₂ and O₂. In addition, according to some embodiments, the conformally coated wire arrays could be used to convert CO₂ and water into methanol. Further, in some embodiments, the conformally coated wire arrays could be used to convert CO₂ into methane or other hydrocarbons.

Additionally, within devices in which the conformally coated wire arrays can drive such reactions, the wire array cores could serve any of a number of purposes. For example, in some embodiments, the wire array core could serve as an inert scaffold for the conformal coating, in which the wire array core serves only as a growth template and all useful light absorption and carrier extraction into solution or to complete the circuit occurs in the conformal coating. Alternately, the wire array core could serve as an electrical contact to the conformal coating material, where the wire array core is conductive and is used to extract electrons or holes from the conformal coating. According to some exemplary embodiments, the wire array core and the conformal coating could have “tandem” geometry, in which both wire array core and the conformal coating usefully absorb light and contribute to the photovoltage and/or photocurrent used to drive chemical reactions.

According to other embodiments of the present invention, systems and devices using the photoelectrodes described above are provided. Such systems or devices are useful for many applications, such as, for example, the generation of fuel, for example solar fuel (e.g., syngas) or H₂. In some embodiments, as shown in FIG. 7, the device includes a photoanode and a photocathode separated from each other by an electronically and ionically permeable, and proton-conductive membrane. In some embodiments, at least one of the photoanode or photocathode includes one of the photoelectrodes described above having a wire array core and a conformal coating on the wire array core. According to some exemplary embodiments, both the photoanode and photocathode comprise a photoelectrode having a wire array core and a conformal coating on the wire array core.

Photoelectrode Geometry

According to embodiments of the present invention, as discussed above, at least one of the photoanode or photocathode has a wire array (or rod) structure or geometry, as depicted generally in FIG. 1 b. As shown in FIG. 1 b, the rod geometry defines a light absorption pathway that is fundamentally different from the pathway of conventional planar geometries. In particular, in conventional planar photovoltaic or photoelectrochemical junctions, the directions of light absorption and of charge-carrier collection are mutually parallel, as shown in FIG. 1 a. Charge-carriers generated deep within the bulk of the absorbing semiconductor material must therefore necessarily traverse the full thickness of the absorber to be collected. As such, the requisite long minority-carrier diffusion length, L, explicitly demands high material purity to minimize bulk carrier recombination, and thus implicitly requires significant materials preparation and purification costs.

In contrast to the conventional planar junctions, photo-sensitive junctions having rod geometries according to embodiments of the present invention, employ radial charge-carrier collection, and can therefore exhibit good energy conversion efficiencies even with relatively low-cost, low-grade bulk materials. Importantly, impure semiconductor materials are much more amenable to scalable, inexpensive manufacturing and processing methods as compared to expensive single crystals. The bulk purity constraints of materials employing a radial charge collection strategy (e.g., materials with rod geometries) are relaxed due to the orthogonalization of light absorption and charge carrier collection, as depicted in FIG. 1 b. As long as the radius of the high aspect ratio rod electrode is no greater than the value of L in the bulk of the semiconductor, photogenerated charges can be collected efficiently.

This behavior has recently been demonstrated for Si wire arrays (as shown in FIGS. 2 a, 2 b, 3 a, 3 b and 3 c) and for CdSeTe wire arrays (i.e., nanorods). For CdSeTe nanorods, the spectral responses indicated that the quantum efficiency of the rod array electrode remained relatively constant up until the band gap energy while the quantum efficiency steadily decreased with longer wavelength incident radiation on the planar electrode samples (see FIG. 4). Hence, the rod array electrode exhibited improved collection of photogenerated charge-carriers even for excitation with long penetration-depth photons. Results to date from the J-E measurements have shown that these rod electrodes are capable of achieving short-circuit current densities similar to those of the planar electrodes.

A second key advantage of this geometry is that the ratio of actual junction area to projected area can be large in a high aspect ratio rod array. For a constant illumination intensity, the flux of charge-carriers through a given area element can be significantly lower than it would be at an analogous area on a planar electrode (see FIG. 5). As such, the demands of the electrocatalyst to support the photogenerated charge-carrier flux are much less. Assuming the ratio of actual-to-projected electrode area is on the order of 10², this allows the use of catalysts with a hundred-fold less turnover number to still efficiently produce fuel with the available solar photon flux.

A third advantage of this geometry is that it allows use of two separate absorber materials, one for the photoanode, to obtain stability in oxidizing conditions, and the other for the photocathode, to obtain stability under reducing conditions. A band gap of about 2.0 to about 2.6 eV is optimal for a cell using a single absorbing semiconductor whose band gap straddles the electrochemical potentials necessary to electrolyze water, or reduce CO₂. A semiconductor with a bandgap of 2 eV could absorb approximately 8×10¹⁶ photons cm⁻² s⁻¹ (Air Mass 1.5 solar spectrum at 100 mW cm² of insolation), producing a current of 14.6 mA cm⁻² (100% quantum efficiency) and potentially yielding up to 7.6×10⁻⁸ moles of CO/H₂ cm⁻² s⁻¹. The maximum efficiency that could therefore be obtained is equal to 18% (1.23 eV×14 mA cm²/100 mW cm⁻²). If the band gap of the material is 2.6 eV, the limiting efficiency is approximately 5%. However, such a cell only uses a quarter of the photons available in the solar spectrum. The solar spectrum has approximately the same number of photons with energies of >about 2 eV, between about 1.5 and about 2 eV, and between about 1.2 and about 1.5 eV. Thus, the use of multiple photons is possible without a loss in efficiency and with possible increases in efficiency. Hence, use of both a photoanode and photocathode enables each band gap (about 1.1 to about 1.4 eV) to be better matched to the solar spectrum (see FIG. 6). A 1.2 eV bandgap material could absorb 2.5×10¹⁷ photons cm⁻² s⁻¹; since there are two absorbers, 1.25×10¹⁷ electron-hole pairs cm⁻² s⁻¹ would be generated, producing 10⁻⁷ moles H₂ cm⁻² s⁻¹, and a limiting efficiency of over 25%. The limiting CO/H₂ production efficiency using two nearly optimal band gap electrodes will therefore be greater than that available using one large band gap material. In addition, using two photoelectrodes allows for greater flexibility in materials and design, rather than relying on finding the one ideal semiconductor as the solar absorber. To achieve high photoconversion efficiency with a broad spectral absorption, an optimal photoelectrode structure would consist of several light absorbing layers, each independently connected, and absorbing a different portion of the solar spectrum, but with an overall charge separation potential in each layer exceeding 1.23 V. Thus, the top layer might consist of a single junction high bandgap (˜2.0 eV) material and the underlying layers might include two-junction photoelectrodes consisting of same-bandgap tandem structures (e.g., 1.1 eV/1.1 eV) or different bandgap tandem structures (e.g. 1.4 eV/0.7 eV or 1.1 eV/0.7 eV).

According to embodiments of the present invention, two separate, photosensitive semiconductor/liquid junctions are used that collectively generate about 1.7 to about 1.9 V at open circuit, and about 1.2 to about 1.4 V at maximum power, which is necessary to support both the oxidation of H₂O (or OH⁻) and the reduction of H₂O and CO₂. The photoanode and/or photocathode include rod-like semiconductor components, with controlled, bonded heterogeneous multi-electron transfer catalysts, as needed to drive the oxidation or reduction reactions, respectively, at low overpotentials. The high aspect ratio semiconductor rod electrode architectures serve to orthogonalize the directions of light absorption and charge-carrier collection, thereby substantially relieving the constraints on purity (and material costs) required of the semiconductor, and simultaneously enabling efficient absorption of sunlight while also facilitating effective collection of photogenerated charge-carriers over relatively short distances. Additionally, the high surface-area design of the rod-based semiconductor array electrode inherently lowers the flux of charge-carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any needed electrocatalysts to perform the water-splitting half-reactions in the cell.

In some embodiments of the present invention, as discussed above, a device includes a photoanode and a photocathode separated by a membrane. At least one of the photoanode and photocathode is the photoelectrode as described above including a wire array core and a conformal coating on the core. In some embodiments, for example, a device includes a conformally coated wire array (e.g. a GaP coated Si wire array) as both the photoanode and the photocathode. In such an embodiment in which GaP is used as the conformal coating, n-type GaP could be used as the coating for the anode, and p-type GaP could be used as the coating for the cathode. In some alternate embodiments, the device could include a conformally coated wire array as the photocathode (e.g., p-type GaP on Si wires), and different material, either with a planar or wire array geometry (e.g., porous WO₃), could be used as the photoanode. In yet other alternative embodiments, the device could include a conformally coated wire array as the photoanode and different wire array (not coated) as the photocathode (e.g., an uncoated Si wire array). Alternatively, some embodiments of the present invention are directed to devices in which the conformal coating material serves as the photoanode (e.g., n-type GaP or WO₃) or photocathode (e.g., p-type GaP or a GaAs_(x)N_(y)P_(1-x-y) alloy) for solar fuel generation.

Photoelectrode and Membrane Materials

A key constraint to the development of the photoelectrodes (i.e., the photoanode and photocathode) for the inventive devices is that there is no known semiconductor material that is stable to both oxidizing and reducing conditions simultaneously, and which has a sufficiently small band gap to allow effective absorption of a large part of the solar spectrum. Although metal oxides are generally oxidatively stable, their conduction bands do not provide sufficient reducing potential to produce fuel in the reductive half-reactions required for syngas production. Importantly, metals generally require large overpotentials to reduce CO₂ to CO, and hence, without adequate electrocatalyst development, a large portion of the photovoltage produced by a photovoltaic module will be wasted/needed to overcome the prohibitively large kinetic barriers to the chemical reactions involved with the fuel-production process.

Significantly, one material, that also happens to be photoactive, stands out uniquely to date: p-GaP. This semiconductor has been shown to allow for the reduction of CO₂ in water, with high current efficiency and at low overpotential. In fact, recent work has shown that the process proceeds, through use of light, at potentials less negative than the thermodynamic potential for the reduction process at a reversible, non-photoactive metal electrode. This system therefore represents a true energy-storing photoactive material that robustly produces, and electrocatalyzes, the reduction of CO₂ at ambient conditions capable of producing CO (and H₂, if desired, at more negative potentials).

As it stands, however, p-GaP does not produce, by itself, sufficient photovoltage to perform the required oxidation of H₂O to produce O₂, and therefore a full sustainable production of CO and H₂ with the only inputs being sunlight, CO₂, and H₂O, is difficult to achieve with this material alone as it is difficult to achieve a high enough voltage. Furthermore, GaP, like most non-oxide semiconductors, is oxidatively unstable in water, forming Ga³⁺ ions and phosphate under anodic conditions. In addition, due to the constraint in conventional planar solar cell designs for long diffusion lengths, and thus highly pure material, very pure, long diffusion length, expensive, p-GaP single crystals are required to obtain respectable quantum efficiencies for CO and H₂O reduction to allow effective charge carrier movement, separation, and collection over a distance comparable to the absorption length in the indirect band gap semiconductor p-GaP.

However, embodiments of the inventive devices circumvent all of these limitations and represents the first true artificial photosynthetic system that produces syngas from H₂O, CO₂, and sunlight as the inputs. In some embodiments, for example, the systems for solar fuel generation include membrane-bridged structures with p-GaP and n-WO₃ as the functional photocathode and photoanode materials, respectively. In some embodiments, the photocathode can include a Si wire array core conformally coated with a p-GaP layer, and the photoanode can include a WO₃ material either in a planar or wire array geometry.

The photoanode and photocathode components are electrically and ionically interconnected through, but physically separated by, the membrane, e.g., a flexible composite polymer film. Multi-component membranes, composed of existing polymeric materials, that exhibit the mechanical pliability, electronic conductivity, and ion permeability properties necessary for a feasible water electrolysis system may also be used. Specifically, polypyrrole can be used to make electrical contact between the anode and cathode, while poly(dimethylsiloxane) (PDMS) can be used to provide structural support for the semiconductor rod arrays. For proton conduction in a cell operated under acidic conditions, Nafion can be employed, whereas vinylbenzyl chloride modified films of poly(ethylene-co-tetrafluoroethylene) (ETFE) can be used for hydroxide conduction in a cell operated under alkaline conditions. Hence, embodiments of the present invention are directed to a feasible and functional prototype and blueprint for an artificial photosynthetic system composed of only inexpensive, earth-abundant materials, that is simultaneously efficient, durable, manufacturably scalable, and readily upgradeable.

Photocathode

The photocathode material in the semiconductor/liquid junction device must satisfy several requirements. The conduction band-edge position must be sufficiently negative of the formal potentials to produce hydrogen and/or CO from water that the reaction between conduction band electrons and solution species is energetically favorable (see FIG. 8). A suitable photocathode must also be stable under the reducing environment encountered in the operating conditions of the cell. Finally, the kinetics of H₂O or H⁺ reduction must not be rate-limiting.

P-type GaP is well-suited as a photocathode in the water-splitting devices of the present invention. Specifically, p-type GaP is cathodically stable under illumination in aqueous conditions, is documented to reduce, at low overpotential, both CO₂ and H₂O to form CO, CH₃OH and H₂(g) under illumination, and is capable of being coupled with inexpensive yet effective catalysts in ways that do not induce deleterious Fermi level pinning that would lower the photovoltage of the cathode. In developing certain embodiments of the inventive photocathode, methods were developed to use earth-abundant metals as suitable catalysts, and surface passivation was used to overcome the higher ratio of surface area to projected geometric area of the rod-array photoelectrode geometry shown in FIG. 9.

According to some embodiments of the present invention, the nanorods are grown with the desired length and diameter by use of a template, as has been done previously for CdSeTe and Si nanorods. The use of alumina, block copolymer, or polycarbonate membranes allows for a wide selectivity of rod diameter (about 10⁻⁸ m to about 10⁻⁶ m), density (≦about 10¹⁰ cm²), and length (about 10⁻⁶ m to about 10⁻⁵ m). Following electrodeposition, the template is removed by an appropriate chemical or thermal treatment. For alumina templates, a brief soak in strongly alkaline solutions dissolves the alumina. Copolymer and polycarbonate membranes can be thermally decomposed in an oxygen-rich ambient. These techniques were used to make a variety of different p-GaP nanorod arrays.

As discussed above, according to certain embodiments of the present invention, photoelectrodes include a wire array core and a conformal coating on the core. In some embodiments, the photocathode can include a Si wire array core conformally coated with GaP. Epitaxial growth of high quality GaP on Si is possible because of the small lattice mismatch between these materials. According to embodiments of the present invention, GaP can be grown on arrays of high aspect ratio Si wires by organometallic vapor phase epitaxy (OMVPE), since OMVPE enables highly conformal growth, which is important to ensure full coating of the photoanode by GaP. In order to prepare a clean Si surface for GaP growth, an approximately 200 nm conformal Si buffer layer can first be grown on the Si wires in the GaP growth reactor via growth from silane at 850 C. Following homoepitaxial Si buffer layer growth, a GaP nucleation layer can be grown at about 450 C. with continued GaP growth at about 650 to about 700 C. using tertiarybutyl phoshine (TBP) and triethyl gallium (TEGa) precursors.

The band-gap of GaP is somewhat higher than an optimal value, however, for solar capture and conversion in a tandem absorber configuration. Accordingly, in some embodiments of the present invention, the p-GaP alloy can be doped to enable tunability of the band gap. For example, in some embodiments, p-GaAs_(x)P_(1-x) alloys or p-GaAs_(x)N_(y)P_(1-x-y) alloys can be grown and used as cathodes for CO₂ and H₂O reduction. Such a p-GaAs_(x)P_(1-x) alloy system allows tuning of the band gap throughout the visible region down to the 1.4 eV band gap of GaAs, and provides strong absorption for alloys having x<0.45 due to the change in optical properties from an indirect gap to a direct gap below this P content level in the GaAs_(x)P_(1-x) alloy, and substantially preserves the Ga-based electrocatalytic properties towards CO₂ reduction of the GaP surface. Since the radial junctions provide a large degree of strain relief, if necessary a very thin (insufficient to act as a minority carrier tunneling barrier) annulus of p-GaP can be grown around the lower band gap p-GaAs_(x)P_(1-x) (or GaAs_(x)N_(y)P_(1-x-y)) rods to maintain the desired electrocatalytic properties of the surface that contacts the electrolyte while maintaining the light absorption properties of the core lower band gap GaAs_(x)P_(1-x) material.

Photoanode

To make syngas solely from sunlight, CO₂, and water, O₂ must be simultaneously evolved from the aqueous solution. Several criteria are necessary for sustained and efficient oxidation of water to O₂ at an illuminated semiconductor electrode. From a thermodynamic perspective, the valence-band edge of an n-type photoanode material must be sufficiently positive for the photogenerated hole to have the requisite oxidizing power to effect water oxidation (see FIG. 8). For this reason, most small band-gap semiconductors under illumination, e.g. Si and GaAs, are incapable of supporting water oxidation without additional electrical bias. In contrast, metal oxide semiconductor materials are generally well-suited to serve as photoanodes, because their valence-band edges are sufficiently positive of E°′(O₂/H₂O). The photocathode also has kinetics-based requirements to work effectively. Because the light-driven water oxidation half-reaction necessarily accumulates holes at the semiconductor/liquid interface, the semiconductor surface must be kinetically resistant to deleterious surface oxidation and/or fouling processes. Most metal oxides exhibit prolonged stability against surface photodegradation because the constituent metal atoms are already in their highest oxidation state and are fully coordinated with oxygen. For example, metal oxides such as SrTiO₃, TiO₂, and Fe₂O₃ can operate almost indefinitely under illumination when immersed in an aqueous electrolyte. Finally, the demands on the electrocatalytic activity of the semiconductor interface are significant, as water oxidation is a complex, multi-electron, multi-proton redox process. In the absence of sufficient electrocatalytic activity, water oxidation requires a substantial overpotential to proceed at an appreciable rate. The best performing abiotic water oxidation electrocatalysts are metal oxide surfaces and metal oxide clusters/colloids.

Despite the thermodynamic and kinetics considerations favoring metal oxides, the development of any single metal oxide semiconductor as a practical photoanode has been largely hampered by two factors. The band gaps, E_(g), of most metal oxides, e.g. TiO₂, SrTiO₃, are indirect and large (>3 eV). Hence, most simple binary metal oxide materials do not appreciably absorb the majority of the solar spectrum, especially in thin film forms. Additionally, nearly all metal oxides possess poor charge-carrier mobilities, making efficient collection of photogenerated holes from within the bulk impossible in traditional planar cell geometries. Both the inability to absorb sunlight effectively and the inability to sustain long-lived photogenerated charge-carriers are serious impediments that greatly limit the energy conversion efficiency of a working photoanode.

According to embodiments of the present invention, WO₃ can be used as a photoanode in embodiments of the inventive device. WO₃ can absorb a significant fraction of the solar spectrum (E_(g)=2.6 eV). For a metal oxide, WO₃ also possesses comparatively long charge-carrier diffusion lengths. The charge carrier diffusion lengths in crystalline WO₃ can be as large as 10⁻⁶ m,³⁷ compared to about 10⁻⁸ m for α-Fe₂O₃, another metal oxide that has received much more attention as a possible photoanode material. Use of a rod array electrode architecture lessens the required charge-carrier diffusion lengths, making WO₃ a viable photoanode material. Furthermore, both of these key material aspects of WO₃ can be altered, tuned, and possibly enhanced by alloying with another metal. For example, the band gap of WO₃ has been shown to be lowered by several hundred meV by the addition of small fractions of Mo. Alternatively, the band gap of WO₃ can be decreased by alloying with nitrogen.

According to embodiments of the present invention, a modular rod-array photoanode system (see FIG. 10) utilizes WO₃ as the photoanode material. A rod array electrode architecture will maximize the absorption of visible light and maximize the efficiency of collection of photogenerated holes in WO₃. To prepare rod arrays of n-WO₃, a general template-assisted growth method can be employed, utilizing porous alumina oxide, block co-polymer, or polycarbonate templates. Collectively, these template types can either be obtained commercially or can be readily prepared with size features that allow systematic variation in the rod arrays prepared using these methods.

Arrays of metal oxide rods can be prepared via electrodeposition, through porous templates, onto metallic support substrates (e.g., In-doped SnO₂ or glassy carbon, etc.)(see FIG. 11). According to embodiments of the present invention, exemplary arrays can be prepared via controlled electrodeposition of ‘pure’ WO₃ or of WO₃ alloyed with either Ni or Mo (i.e., W_(1-x)M_(x)O₃ in which M is Ni, Mo or a combination thereof). Cathodic electro-deposition of metallic W, and subsequent thermal oxidation, enables the use of alumina membranes as templates. Using this technique, the Ni or Mo salt solution concentration can be varied and the quantity and distribution of incorporation into the W rod can be measured. Oxidation can be carried out by heating in air or in a controlled oxygen atmosphere. This method is shown on the left side of FIG. 11. As shown in FIG. 11, this method of electrodepositing WO₃-alloy rod arrays generally includes electroplating metallic W alloys, followed by template removal and metal oxidation.

Alternatively, hydrated WO₃ films can be cathodically electrodeposited from peroxo-polytungstate solutions. Simultaneous deposition of Ni or Mo salts in an oxidizing environment can be used to produce alloys. This method is shown on the right side of FIG. 11. As shown in FIG. 11, this method of preparing WO₃-alloy rod arrays generally includes directly electrodepositing hydrated mixed metal oxides into the template and then annealing to yield the WO₃-alloy rod arrays.

Both of these methods can yield crystalline WO₃ when heated in oxygen-rich environments. These different growth methods may result in materials with varying charge-carrier diffusion lengths, a property that can be monitored to optimize the growth process. Two different methods can be employed to monitor this diffusion length. Near-field scanning optical microscopy (NSOM) allows direct quantification of the charge-carrier diffusion lengths, while spectral response measurements will relate the charge-carrier collection efficiency to the penetration depth of the various wavelengths of light.

While both electro-deposition methods will allow the mole fractions of either Ni or Mo to be precisely controlled, the latter method (i.e., the method shown on the right in FIG. 11) is also compatible with the use of either a block copolymer or polycarbonate-based template. As for the growth of p-GaP nanorod arrays, the use of alumina, block copolymer, or polycarbonate membranes will allow a wide selectivity of rod diameter (about 10⁻⁸ m to about 10⁻⁶ m), density (≦about 10¹⁰ cm⁻²), and length (about 10⁻⁶ m to about 10⁻⁵ m).

Following electro-deposition, the template will be removed by an appropriate chemical or thermal treatment. For alumina templates, a brief soak in strongly alkaline solutions will dissolve the alumina. Copolymer and polycarbonate membranes can be thermally decomposed in an oxygen-rich ambient. These techniques can be used to make a variety of different WO₃ arrays that can be used in non-aqueous and aqueous electrolytes, and the array properties can be correlated with the resulting electrode performance. The knowledge of the electrical properties of the as-prepared oxides can be used to select the rod array length scales to maximize the possible energy conversion efficiencies of the electrode structures, while variations in the rod electrode parameters can be used to determine the optimum parameters. Using different growth templates, the rod diameter and density can be varied, while the length varies with electrodeposition time. The rod diameter can be tuned to match the charge-carrier diffusion length, while the rod density can be tuned to maximize the current density. Building on recent reports of TiO₂ and ZnO rod arrays in photovoltaic applications, the critical length scales of the electrodeposited WO₃ rod arrays can be tuned to substantially match the material properties of the WO₃.

According to other embodiments of the present invention, the photoanode can include the photoelectrode described above, i.e., a wire array core and a conformal coating on the core. For example, in some embodiments, the photoanode can include a Si wire array core and a n-type GaP conformal coating. Alternatively, in some exemplary embodiments, the photoanode can include a Si wire array core and a WO₃ conformal coating.

Characterization of Conformally Coated Wire Arrays

I. Simulation Measurements

The following simulated measurements are presented for illustrative purposes only, and are not intended to limit the scope and content of the present invention. In addition, the measurement results presented are the result of simulations, and are not intended to imply that actual measurements were taken.

Si wire arrays have recently demonstrated their potential as photovoltaic devices [1-3]. Using these arrays as a base, embodiments of the present invention are directed to multijunction wire array architectures including Si wire arrays with a conformal GaN_(x)P_(1-x-y)As_(y) coating. Optical absorption and device physics simulations were carried out and these simulations show that much of the solar spectrum can be absorbed as the angle of illumination is varied and that an appropriate choice of coating thickness and composition will lead to current matching conditions and hence provide a realistic path to high efficiencies. High fidelity, high aspect ratio Si wire arrays grown by vapor-liquid-solid techniques have been previously demonstrated (see Spurgeon, et al., “Repeated epitaxial growth and transfer of arrays of patterned, vertically aligned, crystalline Si wires from a single Si(111) substrate,” Applied Physics Letters, 93, 032112 (2008), and U.S. Patent Application Publication Nos. 2009/0020150 to Atwater, et al., entitled STRUCTURES OF ORDERED ARRAYS OF SEMICONDUCTORS and published Jan. 22, 2009, and 2009/0020853 to Kayes, et al., entitled STRUCTURES OF AND METHODS FOR FORMING VERTICALLY ALIGNED SI WIRE ARRAYS and published Jan. 22, 2009, the entire contents of all of which are incorporated herein by reference), and according to embodiments of the present invention, conformal GaP coatings are grown on these wires. In some embodiments, these GaP coated wires can serve as precursors to quaternary compound growth. Structural, optical, and electrical characterization of these GaP/Si wire array heterostructures, including x-ray diffraction, Hall measurements, and optical absorption of polymer-embedded wire arrays using an integrating sphere were performed. The GaP epilayers have high structural and electrical quality and the ability to absorb a significant amount of the solar spectrum, making them promising for future multijunction wire array solar cells.

Silicon wire array solar cells have recently demonstrated up to 96% absorption despite less than 5% packing fraction, 2-3% efficiencies in liquid electrolyte, and up to 8% efficiencies in large-area wire array solar cells. Building on these results, embodiments of the present invention are directed to multijunction wire array architectures including Si wire arrays with a conformal III-V semiconductor coating. In some embodiments, for example, a two junction cell has a bottom cell with a band gap of about 1.1 eV and a top cell with a band gap of about 1.7 eV. Silicon and GaN_(x)P_(1-x-y)As_(y) are good candidate materials for a two-junction cell, as they can be grown lattice matched and with the appropriate band gaps, given the proper choice of N, P, and As composition. Two junction cells using these materials could reach a theoretical maximum efficiency of 37% without concentration and 44% with concentration. Because the performance of GaNPAs solar cells is severely limited by short minority carrier diffusion lengths, this material is suited to the wire array geometry, where the directions of light absorption and carrier collection are orthogonalized. Wire arrays also provide effective concentration without the need for external optics as they can absorb nearly all the incident light despite having a 2.8 μm planar equivalent volume.

GaP provides a model system for beginning to explore multijunction wire array solar cells because it is lattice matched to Si, and, while its band gap of 2.26 eV is larger than would be optimal for a dual junction solar cell, it has better minority carrier transport properties than GaNPAs with well understood material properties. Here, absorption and device physics simulations are presented as well as microscopy, electrical, and optical characterization of epitaxial GaP on Si wire array structures.

As the performance of a GaP/Si solar cell will be current limited by the GaP, the light absorption properties of the GaP layer must be understood and optimized. Accordingly, as a starting point, the angular absorption of a two-dimensional GaP on Si “grating” was considered using a commercially available full field electromagnetic simulator (Lumerical FDTD Solutions). A 1 μm thick, 20 μm tall Si core with GaP thicknesses that varied from 0.5 to 2 μm in 0.5 μm increments was considered. The two-dimensional slabs were placed 7 μm apart (see FIG. 12 a, in which the simulation unit cell is outlined by the dotted box). Boundary conditions for the top, sides, and bottom were fully absorptive (PML), periodic, and fully reflecting, respectively. The reflecting layer mimics a back reflector. Optical constants were taken from D. E. Aspnes and A. A. Studna, “Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV”, Phys. Rev. B 27, 1983, pp. 985-1009, the entire content of which is incorporated herein by reference. A plane wave source at a varying wavelength and incident angle was used for excitation. The power absorbed in the GaP was then calculated and normalized to the incident power. As seen in FIG. 12 b, the outer GaP layer absorbed up to 80% of the above-bandgap incident power, with losses primarily due to absorption by the Si core and by reflection, especially at normal incidence where much of the light misses the structures entirely, simply traveling in between the structures and reflecting back out of the grating. The location of the direct bandgap in GaP is evident in the rapid increase in absorption at shorter wavelengths. With the Si wire arrays according to embodiments of the present invention, it has been shown experimentally that losses due to reflection and transmission through the array can be minimized by incorporating antireflective coatings, scattering particles in between the wires, and a back reflector, resulting in peak absorption of 96%. This suggests that with a thick GaP layer of about 2 μm along with appropriate light trapping techniques, almost all the above-band gap incident light in the GaP layer can be absorbed.

A more realistic, full three-dimensional, periodic wire array, is shown in FIG. 13 a. A 1 μm diameter, 10 μm tall Si wire with a 0.5 μm thick GaP shell, a 7 μm pitch, and the same boundary conditions used above was considered. Computational limits forced reduction of the length of the wire and the GaP thickness, and therefore only normal incidence was considered. These conditions gave a lower bound for actual absorption, as typical wires can be up to 100 μm long, have GaP coatings of more than a micron, and have maximized absorption at oblique angles of incidence. As a point of comparison, the exponentially decaying Beer-Lambert absorption of the wire arrays was calculated as well. The results are shown in FIG. 13 b, where relative power refers to the amount of power absorbed in the listed material as normalized to the total incident power. Looking first at the GaP absorption, it can be seen that at 400 nm, where the GaP is fully absorbing, the full field absorption cross section is larger than would be expected from mere geometric, Beer-Lambert considerations. However, moving to 500 nm, the full field value falls significantly below the Beer-Lambert calculation. Examining the Si absorption, it is seen that this loss of power in the GaP corresponds to an increase in the Si absorption; the GaP acts as a lens, channeling light into the higher index Si core. Overall, the Si absorption is greatly enhanced over much of the spectrum due to the GaP shell. Thus, a thick GaP layer should be employed to maximize shell absorption before light is lost to the Si core or a direct bandgap material should serve as the outer layer. While the relative powers are low due to much of the light missing the structure entirely, it is believed that overall absorption could be boosted by employing the light trapping techniques of M. D. Kelzenberg et al., “Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications”, Nature Mat. 9, 2010, pp. 239-244, the entire content of which is incorporated herein by reference.

As GaNPAs compounds have a smaller, direct band gap, according to embodiments of the present invention, an analogous system uses the material properties of GaAs for the shell. As seen in FIG. 14, the shell absorption increases drastically, and once again, the absorption cross section is much greater than the geometric, analytical value. By weighing the simulated relative power absorption at 400 nm by the solar flux between 280 and 450 nm and the other powers by the solar flux in 100 nm bins on either side of the simulated wavelength, the current that would be expected in the shell and core for the two structures was calculated. The GaP/Si wire array generates 0.63 mA/cm² in the GaP shell and 2.60 mA/cm² in the Si core while the GaAs/Si wire array generates 4.20 mA/cm² in the shell and 0.22 mA/cm² in the core. Thus, the two structures fall on either side of current matching conditions and suggest that with an intermediate material and appropriate thickness choices the wire array cell can achieve current matching, and with light trapping techniques, high efficiencies.

For the GaP/Si wire arrays, full three-dimensional device physics models of the structure were constructed that were identical to those used in the three-dimensional optical simulations, discussed above. The device physics modeler Sentaurus was used, which calculates Poisson's equation and the carrier transport equations for a wide range of physical models and conditions. Si radial pn junction wire arrays have been extensively modeled in this format. As a starting point, GaP radial pn junction arrays on Si supports were considered, top incident, Beer-Lambert absorption was considered, the GaP thickness was varied as in our two-dimensional model, and contacts were placed on the outer GaP emitter and either on the Si or GaP base. The location of the base contact was found to have minimal influence on the properties of the cells as the conduction band offset between GaP and Si is small, and a GaP cell with an n-type base with a doping of 1×10¹⁷ cm⁻³ and a p-type emitter with a doping of 5×10¹⁸ cm⁻³ was considered. Additionally, the GaP diffusion length was varied by modifying the lifetime in the material. The surface recombination velocity was assumed to be 0 for this simple model. As demonstrated in FIG. 15 a, optimal efficiencies were found for GaP thicknesses on the order of the material diffusion length, a result also seen in modeling of Si wires. As shown in FIG. 15 b (depicting the effect on open circuit voltage of diffusion length and thickness), while the open circuit voltage falls off directly with diffusion length, having little dependence on thickness, the short circuit current reaches a maximum value when the diffusion length is comparable to the thickness (as shown in FIG. 15 c which depicts the effect on the short circuit current of diffusion length and thickness), as would be expected for a radial junction geometry where generated carriers have to travel the shell thickness to be collected.

As seen in the aforementioned GaP/Si optical modeling, however, the Beer-Lambert model does not provide a realistic description of the optical absorption behavior of the arrays as their features are of comparable size to the excitation wavelength. Thus, optical generation profiles at a variety of wavelengths were obtained from the full field simulations, weighted appropriately by the solar spectrum, summed, and the whole inserted into the device physics simulation. Whereas in FIG. 15 c, the J_(sc) was calculated assuming that all incident light was absorbed in the wires, this more thorough model assumed that the whole wire array was illuminated, and hence much of the light failed to strike the wires and reflected out of the array without being absorbed. For a 10 μm diffusion length, the Beer-Lambert efficiency for a 0.5 μm GaP thickness was calculated to be 0.97% while the full field value was found to be 0.80%, a decrease due to the GaP lensing effect. While the J_(SC) should be boosted through light trapping techniques, photons will still be directed from the shell into the higher index Si core, lowering the shell absorption.

These simulations suggest that a heterostructure wire array should be able to absorb much of the solar spectrum across a range of angles. Additionally, to attain current-matching conditions, the Si core thickness should be minimized while the shell thickness is maximized for indirect, large bandgap compounds, and the opposite track pursued for direct, smaller bandgap compounds. Through bandgap and thickness engineering, a high efficiency, dual junction solar cell should be achievable with a high enough open circuit voltage to drive chemical reactions.

II. Experimental Measurements

The following experimental measurements are presented for illustrative purposes only, and are not intended to limit the scope and content of the present invention.

A template-based vapor-liquid-solid (VLS) growth technique produces high fidelity, high aspect ratio silicon microwire arrays using inexpensive chlorosilane precursors, as discussed in B. M. Kayes et al., “Growth of vertically aligned Si wire arrays over large areas (>1 cm²) with Au and Cu catalysts”, Appl. Phys. Lett. 91, 2007, pp. 103110, the entire content of which is incorporated herein by reference. The wire arrays used in embodiments of the present invention range in height from 10-50 microns and are 1-2 microns in diameter, as determined by the catalyst particle size during VLS growth and growth time. In some embodiments, prior to GaP growth, the silicon dioxide and copper catalyst are chemically etched, and a thin surface layer of silicon containing residual copper catalyst impurities is removed. To coat these wire arrays with gallium phosphide, according to embodiments of the present invention, metalorganic chemical vapor deposition (MOCVD) using trimethyl gallium and phosphine precursors is used. According to embodiments of the present invention, a two-step growth process is used in which a thin nucleation layer is first grown at about 530 C. followed by growth of a thick GaP layer at about 750 C. Depending on the V/III ratio during growth, these layers are either p-type or n-type. P-type layers are grown using a V/III ratio of about 10, and n-type layers are grown using a V/III ratio of about 80. No additional dopants were added during growth. Reference samples including GaP grown on planar Si substrates were synthesized along with each set of wire array samples. The thicknesses of the GaP layers grown on planar Si substrates were 4-5 microns, including a 40 nm nucleation layer, while GaP grown simultaneously on Si wire arrays exhibited 200 nm to greater than 1 micron GaP coatings, depending on the aspect ratio of the Si wire array used as a growth substrate.

The GaP coated wires were characterized used X-ray diffraction, photoluminescence (PL), Hall effect measurements, and both scanning (SEM) and transmission (TEM) electron microscopy. X-ray diffraction measurements were made in ω-2θ and reciprocal space map configurations. Bright field and high resolution TEM were prepared at an accelerating voltage of 300 kV. Samples were prepared for TEM by embedding the wire arrays in epoxy before microtoming 50-100 nm radial and longitudinal layers. PL measurements were performed using a closed-cycle cryostat cooled to 78 K. Samples were excited using the 458 nm line of an Ar-ion laser that was chopped at 10 kHz using an acousto-optic modulator. The emission was passed through a monochromator and focused onto a visible frequency charge-coupled detector.

The growth conditions discussed above resulted in conformal coatings of GaP on Si microwire arrays, as shown in FIGS. 16 a, 16 b, 16 c and 16 d. The GaP coating is conformal and rough, both on the wire array samples and the planar Si substrates, indicating that the roughness of the layer is caused by the polar on nonpolar nature of the epitaxy rather than the nature of the substrate. The cross-sectional SEM images of cleaved wires (FIGS. 16 c and 16 d) reveal that the GaP coating thickness varies along the length of the wires, with the thickest coating at the top of the wires and thinnest on the Si substrate in between wires. This effect is most pronounced in longer wires. The growth morphology of the GaP is typically rough, whether grown on wire arrays or on planar Si substrates including in the same growth run.

X-ray diffraction measurements (shown in FIGS. 17 a, 17 b and 17 c) show that the layers are epitaxial, and that the epitaxial GaP is preferentially oriented in the <111> direction, matching the orientation of the Si wire array substrates. The planar and wire array GaP/Si samples studied showed preferential orientation in the <111> direction according to ω-2θ scans (FIG. 17 a), but several samples showed <220> and <311> peaks as well. The non-<111> peaks are more prevalent in the wire array samples relative to the planar GaP/Si samples. Broken off, nonvertically oriented GaP/Si wires may contribute to the structural disorder in the GaP/Si wire arrays. From the ω-2θ scans, it is estimated that on average, phases oriented in non-<111> directions would account for less than 0.05% of the sample volume if they were uniformly distributed. A reciprocal space map centered around the <111> peak (FIG. 17 c) from a GaP/Si wire array sample has Si and GaP peaks at the predicted locations. The GaP peak is much broader than the Si substrate peak, with a full width at half maximum of 69.5 and 669 arcsec in the 2θ and ω directions, respectively. The narrow peak width in the 2θ direction indicates a well-oriented film, while the broader peak width in the ω direction suggests a high defect density, as is typically the case for polar, III-V materials grown on nonpolar substrates.

TEM images and selected-area electron diffraction patterns taken along the <111> zone axis are shown in FIGS. 18 a-e. TEM imaging shows conformal coating of GaP on Si with an abrupt interface (see FIG. 18 b). Diffraction patterns of the microtomed samples show single crystal Si wires (see FIG. 18 d) and single crystal, epitaxial GaP coatings (see FIG. 18 e). The Si and GaP diffraction patterns share the same symmetry, but the extra peaks in the GaP diffraction pattern are evidence of twinning. These twin defects may contribute to the broadening of the x-ray spectrum in the ω-direction as well. Using TEM imaging in conjunction with EDS, no residual Cu catalyst was found in the Si wires or in the GaP coatings from the VLS growth within detection limits, which is consistent with the long minority carrier diffusion lengths that have been recently measured in silicon wires and suggests that Cu is not a significant impurity in the GaP as well.

Three samples were studied using PL: a GaP coated Si wire array, a planar GaP/Si reference sample, and a commercial Zn-doped GaP wafer obtained from MTI Corporation, and the PL results are shown in FIG. 19. All three samples exhibited broad emission centered around 650 nm. Previous studies have attributed this sub-bandgap luminescence to defect states within the bandgap, including Zn—O pair recombination, as is visible in the Zn-doped commercial wafer. Some research has attributed the 1.9 eV emission peak arising from GaP grown on Si to donor-acceptor pair recombination, where Si on the P site is the deep acceptor and a deep donor has not been unambiguously identified. Impurities such as Zn, O, Si and C could contribute to luminescence in this range.

To understand light absorption in the GaP/Si wire arrays, optical absorption studies were performed using an integrating sphere and wire arrays which have been embedded in a transparent polymer and peeled off the substrate. Two wire arrays were studied: a Si wire array with 1.5 μm diameter, 30 μm long wires in a square array, and a GaP-coated array grown on a Si wire array substrate with the same properties as the bare Si wire array. To find optical absorption, reflection and transmission through the wire arrays was measured as a function of angle of incidence and wavelength. FIGS. 20 a (Si wires only) and 20 b (GaP coated Si wires) show the optical absorption in both wire arrays. The optical absorption is significantly enhanced by the addition of the GaP coating, and nearly 100% absorption was capable of being achieved in the GaP/Si wire arrays even without any light trapping. The absorption enhancement is most likely caused by two factors: the higher fill factor of the GaP coated wires, and scattering caused by the rough GaP surface, evidenced by the enhanced absorption even below the bandgap of GaP. Additionally, the optical modeling reveals the wires' large absorption cross section and the ability of the GaP to channel light into the Si core, providing additional pathways for absorption enhancement over simply geometric considerations.

While the present invention has been illustrated and described with reference to certain exemplary embodiments, those of ordinary skill in the art will understand that various modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present invention, as defined in the following claims. 

What is claimed is:
 1. A device for photoelectrochemical fuel generation, comprising an electrode comprising: a core comprising an array of wires, the wires comprising a first inorganic semiconductor material; and a conformal coating on the array of wires of the core, the conformal coating comprising a second inorganic semiconductor material.
 2. The device according to claim 1, wherein each of the first and second inorganic semiconductor materials is independently selected from the group consisting of Group IV elemental semiconductors, Group IV compound semiconductors, III-V semiconductors, and II-VI semiconductors.
 3. The device according to claim 1, wherein the first inorganic semiconductor material comprises Si, Ge, SiGe alloys, GaP, GaAs_(x)P_(1-x) alloys, GaAs_(x)N_(y)P_(1-x-y) alloys, and WO₃.
 4. The device according to claim 2, wherein the second inorganic semiconductor material is different from the first inorganic semiconductor material and comprises GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃.
 5. The device according to claim 1, wherein the first inorganic semiconductor material is Si, Ge or a SiGe alloy, and the second semiconductor material is GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃.
 6. The device according to claim 1, wherein the first inorganic semiconductor material is Si, and the second semiconductor material is GaP or a GaAs_(x)N_(y)P_(1-x-y) alloy.
 7. The device according to claim 1, wherein the wires of the array have a length of greater than about 100 nm.
 8. A device for photoelectrochemical fuel generation, comprising: a first electrode comprising: a core comprising an array of wires, the wires comprising a first inorganic semiconductor material; and a conformal coating on the array of wires of the core, the conformal coating comprising a second inorganic semiconductor material; a second electrode; and a membrane separating the first electrode and the second electrode.
 9. The device according to claim 8, wherein each of the first and second inorganic semiconductor materials is independently selected from the group consisting of Group IV elemental semiconductors, Group IV compound semiconductors, III-V semiconductors, and II-VI semiconductors.
 10. The device according to claim 8, wherein the first inorganic semiconductor material comprises Si, Ge, a SiGe alloy, GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃.
 11. The device according to claim 8, wherein the second inorganic semiconductor material is different from the first inorganic semiconductor material and comprises GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃.
 12. The device according to claim 8, wherein the first inorganic semiconductor material is Si, Ge or a SiGe alloy, and the second semiconductor material is GaP, a GaAs_(x)P_(1-x) alloy, a GaAs_(x)N_(y)P_(1-x-y) alloy, or WO₃.
 13. The substrate according to claim 8, wherein the first inorganic semiconductor material is Si, and the second semiconductor material is GaP.
 14. The device according to claim 8, wherein the wires of the array have a length of greater than about 100 nm.
 15. The device according to claim 8, wherein the second electrode comprises: a second core comprising a second array of wires, the wires of the second array of wires comprising a third inorganic semiconductor material; and a second conformal coating on the second array of wires of the second core, the second conformal coating comprising a fourth inorganic semiconductor material.
 16. The device according to claim 15, wherein: the first electrode comprises a photoanode, the first inorganic semiconductor material of the first electrode comprises Si, and the second inorganic semiconductor material of the first electrode comprises n-type GaP; and the second electrode is a photocathode, the third inorganic semiconductor material of the second electrode is Si, and the fourth inorganic semiconductor material of the second electrode comprises p-type GaP.
 17. The device according to claim 8, wherein the first electrode is a photocathode, and the second electrode is a photoanode comprising porous WO₃.
 18. The device according to claim 17, wherein the porous WO₃ comprises an array of porous WO₃ wires.
 19. The device according to claim 8, wherein the first electrode is a photoanode, and the second electrode is a photocathode comprising an array of Si wires. 